By default, the FortiClient will start up automatically when starting macOS. This may not always be desirable but fortunately, auto-start can be disabled quite easily.
The FortiClient launch agent is located at /Library/LaunchAgents/com.fortinet.forticlient.fct_launcher.plist. Use any text editor to open and edit this file, but you probably will need to do this using Terminal and using sudo to launch a text-based text editor (e.g. vi or nano).
With the file open, look for these lines:
Change true to false and save the file. Note that updates to the FortiClient may overwrite the change.